The present disclosure generally relates to wood preservatives and methods of treating wood. More particularly, the wood preservatives and methods of treating wood include applying to the wood a composition including a boron-containing compound and carbonated black liquor subsequent to removal of at least a portion of the lignin in the black liquor.
Chromated copper arsenate (CCA), a leach-resistant wood preservative known for exterior application, has been removed from the commercial market voluntarily by its manufacturers in 2003 because of the toxic nature of arsenic and chromium. Since then, there has been a continuing effort to develop suitable alternative systems. A number of alternative, non-arsenical pesticidal treatments containing heavy metals (primarily copper) have been proposed. For example, U.S. Pat. No. 4,929,454 teaches the treatment of wood with a mixture of a copper compound and a quaternary ammonium compound. This technology has been commercialized under the name ammoniated copper quaternary amine (ACQ). ACQ has excellent insect resistance, but it is considerable more costly than CCA, and it has a tendency to promote the growth of white mold on the wood surface. Furthermore, ACQ-treated wood may exhibit corrosion problems with most metal fasteners when the treated wood is placed into service. Special fasteners having high corrosive resistance is required for the ACQ treated-wood, causing an additional cost of using ACQ-treated wood for construction. The industry has since moved to corrosion resistant fasteners such as stainless steel or plastic coatings. Furthermore, there has been increasing concerns on the toxicity and environmental impact of wood preservative containing heavy metals.
Borate has been used as wood preservative for more than 50 years, since it is effective against most wood destroying organisms such as fungi, termite and wood-boring beetles. Furthermore, borate has a low acute mammalian toxicity and low environmental impact. Borate has been considered as an excellent candidate for the CCA replacement for wood preservative application. However, the well-known disadvantage of borate wood preservative is that borate is readily soluble in water, and easily leaches out of the treated wood upon contact with water. As a result, the use of borate preservative is limited to the treated wood for interior applications.
Several methods have been used to prevent the leaching of impregnated borate preservative from the treated wood. U.S. Pat. No. 2,194,827 uses solubilized metal such as zinc and copper to fix borate in wood. This method requires high concentration of ammonia to solubilize such metals and borates, resulting in excessive ammonia volatility and noxious fumes that is undesirable for large scale preparation. U.S. Pat. No. 6,896,908 addresses the ammonia off-gas issue by dissolving a high concentration of copper and/or zinc metal fixative agent in an aqueous solution of ammonia, volatile organic acid and ammonium salts. The combination of a volatile organic acid and ammonia provides a high rate of metal dissolution without requiring excessive levels of ammonia in solution, and the ammonium salt reduces the level of free ammonia needed for dissolution of metals. U.S. Pat. No. 5,207,823 discloses copper borate and/or zinc borate in combination of amine as a leach-resistant borate wood preservative. PCT Patent No. 95/27,600 teaches the use of nitrite to improve fixation of preservatives in wood, when the preservatives contains one or more copper and/or zinc salts of weak acid, and optionally boric acid and quaternary ammonium salt. U.S. Pat. No. 6,146,766 discloses the use of water soluble sodium silicate/borax mixture wherein the impregnated silicate component can be polymerized to reduce its water-solubility, thereby decreasing the leaching rate of water-soluble preservative from the treated wood. U.S. Pat. No. 6,508,869 uses amine oxide to improve leaching resistance of boron preservatives from the treated wood. In U.S. Pat. No. 5,087,457, polyammonium salts formed through the reaction of diamine and dihalide, are used in combination with borate to reduce leaching rate. However, the problem with these methods is that even the most water-insoluble borates, boric esters, and borate complexes will, on prolonged contact with water, hydrolyze to form boric acid which will leach out of the wood.
Therefore, a need exists for improved preservatives and methods for treating wood that overcomes the problems noted above.